This invention relates to the testing of containers such as cans, and in particular to the testing of the integrity and quality of the internal coating of the can.
A well established method of measuring internal coating integrity is the xe2x80x9cenamel rater,xe2x80x9d one example of which is in U.S. Pat. No. 4,206,407. The can to be tested is filled with electrolyte and a DC signal is applied to the can. By measuring the current flow, an indication of the metal exposure can be obtained.
It has also been proposed to measure the thickness of can coatings by pressing a conductive rubber probe against the coating surface, to form a capacitor, and measuring the value of the capacitance to give an indication of the coating thickness. In one arrangement the can is filled with electrolyte rather than using a conductive rubber probe. These testing systems suffer from various disadvantages, the rubber probe method can only give xe2x80x9cspotxe2x80x9d readings from individual areas of the inside can surface, and therefore requires a large number of repeated measurements to give any sort of overall picture of the can coating. The electrolyte gives a general overall indication of the can coating thickness, but it cannot give any sort of indication of distribution of coating material on different parts of the can. The present invention provides an improvement to these types of measuring systems.
Accordingly there is provided a method of measuring the quality of the coating on the inside of a can which comprises the steps of:
i. attaching at least one electric terminal to the can;
ii. inserting an elongate electrode into the can;
iii. applying an electrical signal to either the electrode or electric terminal;
iv. adding an electrolyte to the can; and
v. measuring the current flowing between the electrode and the electric terminal as the level of the electrolyte is being varied in a controlled manner in order to give an indication of the quality of the coating at various parts of the can.
By measuring the current flowing as the level of the electrolyte in the can is being varied, readings representing the coating quality at different parts of the can may be generated. For example, a reading may be generated which represents the coating quality of the base of the can, at various positions up the cylindrical sidewall of the can, or even possibly at the necked-in portion of the can. Level detectors may be employed to associate the timing of the electrical readings with the progression of the electrolyte up or down the can as it is filled with or emptied of fluid. Preferably, the current is measured as the electrolyte is being added to the can.
The method preferably includes the step of applying an ac signal to either the electrode or the electric terminals, and conveniently of measuring the capacitance of the can. As previously mentioned, the capacitance reading may be used to give an indication of the coating thickness, and by measuring the capacitance as the electrolyte level is being varied, the can coating thickness at various positions within the can may be established. This may help to determine, for example, the distribution of coating material within the can as between the sidewall and the base, or between different areas of the sidewall itself.
Conveniently, the method additionally or alternatively includes the step of measuring the phase relationship between the AC voltage signal applied between the electrode and the electric terminal, and the current flowing between the electrode and the electric terminal. This phase shift signal will depend on both the resistance and capacitance values of the can circuit. Ideally, the can circuit will comprise a small resistance derived from the electrolyte in series with a capacitance derived from the dielectric properties of the can coating. Ideally, the phase relationship should vary in a consistent manner as the level of electrolyte in the can is varied. However, if there is a gross disconformity in the can coating, this will result in metal exposure, and a conductive path through the coating which will significantly alter both the can circuit and the phase relationship. In this way, not only the presence but also the approximate location of any such disconformities can be detected. Similarly, any problems with the electrical connection between the electric terminal and the can will affect the resistance of the circuit and, hence, the phase shift signal.
The current flowing is preferably converted to an analogue AC voltage by an appropriate electronic circuit, which itself will introduce a phase shift. A convenient method of measuring a parameter which will be indicative of the integrity of the can coating is to apply the AC signal to a fixed circuit comprising a resistor and capacitor in series, and converting the current to an analog AC voltage using an electronic circuit identical to that used for conversion of the can current. The phase difference between the two analogue voltages can then be measured, and it is the way in which this parameter varies as the electrolyte level in the can is varied which indicates whether or not the can coating is of acceptable integrity, and if not, the magnitude and position of defects.
Preferably the method includes the step of interpreting the measured current in terms of the dry film weight of the can coating. To do this it is necessary to have a calibration factor for the particular coating being used, which calibration factor is used to relate the capacitance value to the dry film weight of the coating on the can. To determine this calibration factor a gravimetric measurement of the cans is made (i.e. weighing the cans before and after the coating has been applied), and the film weight measured is compared to the capacitance values obtained. In this way the capacitance values can be directly interpreted into dry film weight readings for that particular coating material.
The invention further resides in apparatus for measuring the quality of a coating on the inside of a can, the apparatus comprising
i. at least one electric terminal adapted to be attached to the can;
ii. an elongate electrode adapted to be inserted into the can;
iii. a power supply for applying an electrical signal to either the electrode or the electric terminal;
iv. means for adding an electrolyte to the can and varying the level of the electrolyte in the can at a controlled rate; and
v. means for measuring the current flowing between the electrode and the electric terminal as the level of the electrolyte is being varied, in order to give an indication of the quality of the coating at various parts of the can.